The low angle X-ray diffraction pattern of rat tail tendon gives a set of forty or more meridional reflections of 670 A spacing, reflecting the staggering of the collagen molecules in the axial direction; and a set of equatorial lattice reflections indicative of the molecular packing in the fibril. The goal of the research proposed here is to understand the molecular arrangements in different connective tissues by extending low angle diffraction studies to investigate tissues with different genetic types of collagens, with different amounts and kinds of non-collagenous components, and with abnormalities, and to examine fibrils reprecipitated from collagen solution. The axial arrangements of the fibrils will be examined by recording the intensities of the meridional reflections and then interpreting the differences in relative intensities between samples in terms of electron density differences. The recent success in relating the electron density distribution in a D period of rat tail tendon (as derived from the amino acid sequence and D stagger) to the observed meridional intensities provides a basis for interpreting this work. The three-dimensional packing of the molecules in the fibril will be examined with electron microscopy (after using X-ray diffraction to find specimen preparation procedures that will not disrupt the original structure) in correlation with X-ray diffraction on stained tendons and other tissues. Such studies should allow one to define the different arrangements of collagen molecules in different tissues and in diseased states, and to understand the basis of these different arrangements in terms of the genetic types of collagen and the presence of non-collagenous components.